Bovine leukemia virus (BLV), a RNA C-type virus induces syncytia in bovine embryonic spleen cell monolayers, and provides a sensitive assay for the detection and quantitation of BLV. We have adapted this assay to micro system, which is at least as sensitive as the standard assay and more economical. BLV can be isolated from peripheral blood lymphocytes (PBL) of a BLV infected animal. However, lymphocyte subpopulation carrying BLV is unknown. We separated PBL from BLV infected animals on nylon wool columns into B-enriched and B-depleted populations and analyzed for BLV by electron microscopy and syncytia induction assay. Our studies demonstrate that BLV is associated with B-enriched lymphocyte population. Lymphocytic nuclear pockets associated with bovine leukemia were also present in the B lymphocytes. We have also developed a marker for Fc receptors and demonstrated a population of lymphocytes bearing Fc receptors in the bovine. Double labeling experiments suggest that lymphocytes bearing Fc receptors are a subpopulation of surface immunoglobulin bearing B-lymphocytes.